Higher Order Surfaces in OpenGL with NV_evaluators. Sébastien Dominé

Transcription

1 Higher Order Surfaces in OpenGL with NV_evaluators Sébastien Dominé

2 Why surfaces? Higher order primitives Animation Level of Detail Bandwidth Filtering 2

3 Overview What are the general evaluators defined by NV_evaluators? How do they differ from standard OpenGL evaluators? Important new features Per-edge tessellation control Fractional tessellation Support for all per-vertex attributes Direct support for triangular patches 3

4 What are the general evaluators defined by NV_evaluators? Like OpenGL evaluators, general evaluators can be used evaluate polynomial and rational bivariate functions defined relative to the Bezier basis 4

5 Why New Evaluators? OpenGL evaluators have not seen widespread use due to a variety of problems Difficult to optimize spec too strict on order of evaluation Incomplete support for all vertex attributes (particularly multiple texture coordinates) Difficult to use with adaptive LOD (tessellation scheme too rigid) 5

6 Why New Evaluators? (2) General evaluators address these issues Ease of optimization Support only 4-component maps Evaluate full range of all enabled maps at once Support all vertex attributes Provide more flexible tessellation schemes Eases varying LOD on adjacent patches Provides mechanism for smooth LOD (pop less) Direct support for triangular patches Variable order of polynomial basis 6

7 Triangular Patches are Necessary 3DStudio Max Patch outlines Hardware emulation 7

8 Direct Support for Triangular Patches Triangular patches can be converted to regular patches, but NV_evaluators provides support directly 8

9 OpenGL Entry Points MapControlPointsNV EVAL_2D_NV or EVAL_TRIANGULAR_2D_NV Specify the control points for the vertex component Specify the order of the polynomial basis MapParameterivNV & MapParameterfvNV MAP_TESSELLATION_NV Specify the tessellation factors for the patch EvalMapsNV Tessellate (render) the patch, sending vertices, forming triangles to T&L 9

10 Per-Edge Tessellation Specification Easy to vary the tessellation across a patch Easy to render adjacent patches without cracks 10

11 Per-Edge Tessellation Specification (2) Crack prevention Tessellation factors must be specified consistently for shared edges. Data structures or - Tessellation factors must be based on shared information, patch edges. Criteria Curvature Screen space Transform the control hull edge (Bézier) Silhouettes Shared surface normals 11

12 Four Independent Tessellation Factors

13 Generalized Tessellation is Important 13

14 Fractional Tessellation Not only is each edge s tessellation independent, the tessellation does not have to be integral values! Enable continuous and pop less LOD 14

15 Fractional Number of Segments

16 Level of Detail Calculation screen space edge length By patch significant foreshortening Transform to screen space Compute control hull length Divide by desired edge length By object little foreshortening Beforehand Compute boundary lengths in model space Each frame (or so) Transform Bbox or proxy to screen space Compute size Scale relative lengths computed off-line 16

17 Support for All Vertex Attributes A map can be applied to any numeric vertex attribute (0-15) with values as described below attribute number per-vertex Parameter per-vertex Command 0 vertex position glvertex 1 vertex weights glvertexweightext 2 normal glnormal 3 Primary color glcolor 4 secondary color glsecondarycolorext 5 Fog coordinate glfogcoordext 8 Texture coord 0 glmultitexcoord 9 Texture coord 1 glmultitexcoord 10 Texture coord 2 glmultitexcoord Texture coord 3 Texture coord 4 glmultitexcoord glmultitexcoord 13 Texture coord 5 glmultitexcoord 14 Texture coord 6 glmultitexcoord 15 Texture coord 7 glmultitexcoord 17

18 Support for All Vertex Attributes (2) More flexible: Non implicit texture mapping Non implicit vertex colors Binormal and tangent computation for per pixel bump mapping (see Vertex Programs) 18

19 Skinning & Blending Modify control points on the CPU Simple Recalculate patch every change (frame?) Careful with the continuity for lighting 19

20 Continuity Affected region Control points neighboring boundary Complications Some control points are incident to two boundaries Solution Project onto a plane at the corner 20

21 Tangents, Normals & Bi-Normals Autonormal Standard T&L Driver does the work for you Only use with standard T&L Careful not to forget NORMALIZE for lighting Manual Vertex Program Compute tangents (partial derivatives) Compute normal (cross product) in Vertex Program Partials are simple! Subtract rows and/or columns of control points Triangles too 21

22 Partial Derivatives 22

23 Example 23

24 Example (2) 24

25 Questions, comments, feedback Sébastien Dominé, 25